In communication networks such as those based on Long Term Evolution (LTE) as specified by the Third Generation Partnership Project (3GGP), there are certain data layer functions designed for mass communication with a large number of wireless devices, commonly referred to as “user equipments” (UEs). Some data layer functions are designed for peer-to-peer control of transport channels and for mapping between transport channels and logical channels. Examples of such functions include those used by the Radio Resource Control (RRC) protocol.
According to the Evolved Packet System (EPS) defined by the 3GPP LTE architecture, the radio access network is referred to as an Evolved Universal Terrestrial Radio Access Network (E-UTRAN). The E-UTRAN includes base stations, referred to as eNodeBs (eNBs) that provide E-UTRA user-plane and control-plane protocol terminations towards the UEs. User-plane protocol examples include Packet Data Convergence Protocol (PDCP), Radio Link Control (RLC), Medium Access Control (MAC), and Physical Layer (PHY), while control-plane protocol examples include RRC.
The eNBs are connected by an “S1” interface to a core network, which is referred to as an Evolved Packet Core (EPC). More specifically, the eNBs have S1 connections to a Mobility Management Entity (MME), through an S1-MME interface and to a Serving Gateway (S-GW), through an S1-U interface. Upon request from an MME, an eNB performs an E-RAB to radio bearer mapping and establishes a Data Radio Bearer and allocates the required resources on the air interface, referred to as the “Uu” interface. The eNB also sets up a logical channel for the UE and allocates it to a transport channel. These operations involve the MAC layer.
3GPP specifies the E-UTRAN MAC protocol as a sublayer of layer 2. Functions of the MAC sublayer are performed by MAC entities in the UE and in the E-UTRAN. For a MAC entity configured at the eNB, there is a peer MAC entity configured at the UE and vice versa.
A mapping of logical channels to transport channels at the MAC sublayer is configured by RRC signaling. There is one Logical Channel Identifier (LCD) field for each MAC service data unit (SDU) included in the corresponding MAC protocol data unit (PDU). The LCID field size is 5 bits, where the value 00000 is reserved for CCCH and the value 11111 is reserved for padding. The LCID for the Downlink Shared Channel (DL-SCH) uses the range 11010-11110 for MAC Control Elements (MAC CEs). A MAC CE is an explicit MAC inband control message. The range 01011-11001 is reserved for future needs within the framework of the controlling standard. Similarly, the LCID for the Uplink Shared Channel (UL-SCH) uses the range 11000-11110 for explicit MAC inband control, while the range 01100-10111 is reserved for future needs within the standard.
The LCID values that are predefined for use in identifying logical channels in the MAC sublayer are 00001-01010. From within this range, the LCID values of 00001 and 00010 are reserved for the signaling radio bearers used by RRC. Consequently, there are eight LCID values available for mapping logical channels to data radio bearers.
These and other details can be seen in the following FIGS. 1-4, which include tables excerpted from 3GPP TS 36.321, V12.4.0 (2015-01). In particular, FIG. 1 depicts “Table 6.2.1-1 Values of LCID for DL-SCH”, FIG. 2 depicts “Table 6.2.1-2 Values of LCID for UL-SCH”, FIG. 3 depicts “Table 6.2.1-3 Values of F field”, and FIG. 4 depicts “Table 6.2.1-4 Values of LCID for MCH”.
From the above information and FIGS. 1-4, one sees that in the example context of LTE, there is a relatively scarce set of occupied LCID values within the predefined set(s) of available LCID values. Moreover, one sees that the standard tightly controls the meaning and use of the available LCID values. As a general proposition, conformance to these default meanings or mappings is required for proper operation between the network and the wireless devices. Moreover, to the extent that one might wish to deviate from or expand these default mappings, standardizing new LCIDs for MAC control or other purposes is a slow, cumbersome process.
The transmission of an LCID value from a UE to a network node typically occurs in the context of a random access process used for connection setup, as described below. More specifically, transmission of the LCID value typically occurs within RRC signaling of a Message 3, as described below.
The UE initiates the random access process by transmitting a random access preamble to an eNB via a physical random access channel (PRACH), a transmission commonly referred to as Message 1 (Msg1). In some circumstances, including so-called “contention-free” random access, the transmitted preamble may be assigned by the network. Alternatively, in other circumstances, including so-called “contention-based” random access, the transmitted preamble may be randomly selected by the UE from one of multiple possible groups of preambles (e.g. preamble groups A and B).
In contention-based random access, selection of a preamble from a particular group may be used to convey information about the amount of data the UE desires to transmit in subsequent transmissions. For example, selection of a preamble from group A may indicate that the UE desires to transmit a relatively low amount of data, while selection of a preamble from group B may indicate that the UE desires to transmit a relatively high amount of data, or vice versa.
Once a preamble has been transmitted by the UE and detected by the eNB, the eNB transmits a Random Access Response to the UE on a downlink shared channel (DL-SCH), a transmission commonly referred to as Message 2 (Msg2). Thereafter, the UE transmits an RRC connection request message to the eNB on an uplink shared channel (UL-SCH), a transmission commonly referred to as Message 3 (Msg3). The UE and eNB then communicate using the RRC protocol to establish a connection for data transport between the UE and the network.
Once the UE is connected to the eNB, the eNB facilitates data transport between the UE and the network. In general, the data transport performance may be limited by the radio access capability of the UE, which may be indicated by a UE classification or some other mechanism. Different types of UEs, for instance, may have different radio access capabilities as defined in 3GPP TS 36.306, such as number of receive antennas, maximum number of layers for uplink transmission, maximum data rates in uplink and downlink.
The eNB generally needs to know UE capabilities in order to properly assign resources, perform control functions, and conduct communication with the UE, among other things. The eNB may obtain information regarding UE capability in any of several ways.
In a typical approach employed in E-UTRA, for example, a UE conveys its capability to the eNB in an RRC UECapabilityInformation message as defined in 3GPP TS 36.331. This information is typically sent upon request from the eNB, after an RRC connection is established.
In an alternative approach, a Category 0 UE, as defined in 3GPP TS 36.306, has restrictions on transport block size, which requires UE capability to be known by the eNB before the RRC connection is established. A Category 0 UE therefore signals its category in the random access procedure, more specifically using a particular logical channel identifier (LCD) in Random Access Msg 3, as defined in 3GPP TS 36.321.
In yet another alternative approach, an even earlier communication of UE capability is proposed. For 3GPP Rel-13, a coverage-limited UE can convey its so-called PRACH repetition level by the random access preamble that it transmits, as described in 3GPP TR 36.888 and 3GPP Tdoc R1-150920. In other words, the UE capability can be identified based on the initial transmission of the random access preamble. To that end the preambles in a cell are partitioned into more groups than preamble groups A and B and the group of preambles for contention-free random access. The network can then apply relevant coverage-enhancement schemes already from the random access response. Coverage-limited UEs may repeatedly transmit the same preamble in several PRACH both to be detected and to be classified by the eNB.